The present invention relates to a golf ball material which has a good thermal stability, flow and processability and from which there can be obtained high-performance golf balls having excellent properties such as rebound and durability. The invention relates also to a golf ball which includes as a component therein a molded part made from such a golf ball material, and to a method for preparing such a golf ball material.
In recent years, ionomeric resins have been widely used in cover materials for golf balls. Ionomeric resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, in which some of the acid groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics in terms of durability, rebound and scuff resistance of the ball.
At present, the base resins used in golf ball cover materials are generally ionomeric resins, but various improvements are being made to address the constant desire by players for golf balls having a high rebound and an excellent flying performance.
For example, cover materials composed of an ionomeric resin to which a large amount of a metallic soap has been added have been disclosed (Patent Document 1: U.S. Pat. No. 5,312,857; Patent Document 2: U.S. Pat. No. 5,306,760; Patent Document 3: International Application WO 98/46671) as ways of improving the rebound resilience and the cost characteristics of ionomer cover materials
However, the metallic soap in these prior-art cover materials undergo decomposition and vaporization during injection molding, generating a large amount of fatty acid gases. As a result, molding defects tend to arise and the gases that have formed deposit on the surface of the molded part, markedly lowering its paintability. Moreover, such cover materials do not differ to any greater extent in their rebound resilience from ionomer covers of the same hardness that contain no metallic soap, having a rebound resilience which is either about the same or at best exhibits some modest improvement due to inclusion of the metallic soap, but in any case is not markedly improved. In addition, depending on the type of metallic soap used, significant declines in processability and resilience sometimes occur, making the cover material entirely unfit for practical use.
An ionomer that has recently been developed for use as a golf ball material is a homogeneous-phase, high-rebound-resilience material having an interpenetrating polymer network (IPN) structure (U.S. Published Patent Application No. 2004/0044136). The ionomer is prepared by blending a first component such as an ethylene-(meth)acrylic acid copolymer with a different type of thermoplastic resin as a second component to form a resin composition, then adding as a third component a metal ionic species to neutralize the acid groups on the first component dispersed in the resin composition. However, in this method, since a solid (i.e., a powder or granular material) such as a metal oxide, metal hydroxide or metal carbonate as the metal ionic species is directly used as it is, and also in case of a high acid content in the first component, the addition of a large amount of the metal ionic species is required for neutralizing the acid, during mixing the metal ionic species with the resin components, there are concerns about poor dispersion of the solid metal ionic species in the resin components, leading to plugging of the screen pack attached to the breaker plate on the extrusion die (and thus necessitating interruptions in production), or leaving some of the metal ionic species unreacted. In addition, given that a partial acid-neutralizing reaction (incomplete degree of neutralization) occurs and the target degree of neutralization cannot be achieved in a single-step reaction through one-pass extrusion, more than one pass be done, which concerns about lowering the physical properties of the resulting ionomer composition.